When a person thinks of aircraft with folding wings, probably what most often comes to mind are fighter aircraft that are stationed on board aircraft carriers. Carrier-based aircraft must, by necessity, have folding wings that reduce their wing span for storage purposes. It goes without saying that an aircraft carrier has a small amount of flight deck and hanger space in comparison to ground-based facilities. Providing folding wings on carrier aircraft enables a larger number of planes to be based on a carrier than would otherwise be possible.
Because the same kinds of space considerations have not, at least in the past, applied to the utilization of commercial passenger aircraft or jets, there has been little or no need to have folding wings on these kinds of planes. In other words, the typical airport served by a passenger jet generally has plenty of space for aircraft maneuvering on the ground, and for parking and passenger ingress and egress. However, for reasons stated below, this may now be changing.
It is probably a fair statement to say that modern-day airline operation is linked to passenger demographics or ridership levels, and the ability of the commercial airlines to minimize the number of empty seats on board their various flights when moving passengers from one location to another. The number of people travelling a given route, during a given period of time, dictates the number of scheduled flights provided, and the size or model of jet that is used in providing such flights. Some airplane models are better suited for use on certain routes than others. In other words, a small jet may be better suited for a given route, taking into consideration the number of people who schedule flights along that route during a certain time period, while a larger jet may be more optimum for other routes. It appears that the general trend, given present ridership levels, is to use larger planes, and consequently, the airlines are increasing the size of the individual aircraft in their fleets.
Aircraft size alone, however, is not the only driving factor governing how to move a given passenger volume from point A to point B in the most efficient manner. The major airlines typically have a fixed number of passenger gates at each airport. As is well-known, many, or perhaps most, major airports in the United States have either been built or renovated during the last twenty or thirty years. During such building process, passenger gates were distanced from each other in a fashion that was generally considered to be optimal given the size and number of aircraft in use at the time. The problem with the present trend toward using larger aircraft is that they have much larger wing spans than the majority of aircraft in use twenty or thirty years ago. This results in a lesser number of airplanes taking up a fixed amount of parking or berth space around the typical terminal, and consequently, reduces the number of usable passenger gates. The end result is that larger airplanes tend to make it more difficult to accommodate airline scheduling that calls for a large volume of flights.
Consequently, it now appears that space restriction is becoming a problem in modern-day airports. One way to address this problem is suggested by the military aircraft carrier mentioned above. That is to say, more planes can be fit into a limited space if they are provided with folding wings.
It is well-known that The Boeing Company, who is the assignee of the invention disclosed and claimed here, is presently developing a new model of commercial jet designated as the "777." This jet is larger than Boeing's earlier "767" model, and consequently, has a larger wing span. At least one study done by Boeing estimates that there are approximately one hundred thirty usable gates at six major airports for the Boeing 767 and the DC 10--10. However, only twenty or so of these same gates are suitable for use by the 777, because of its larger wing span.
Providing 777 jets with folding wing tips can enable a larger number of this particular airplane model to be parked or placed around a given terminal at the same time. It has been estimated that nearly as many usable gates would be available for a 777 that is modified in this way as are presently available for the 767 and DC 10.
A 777 with folding wing tips will have a latching and locking system for releasably interlocking the wing tip with the inboard portion of each wing. Safety considerations dictate that such system must have the utmost structural integrity and fail-safe operation. As the skilled person would realize, an aircraft with folding wings presents additional design and safety problems that are not present on aircraft with conventional wings. It goes without saying that the wings must be designed so that they do not accidentally fold during flight, and that the latching and locking mechanism does not falsely indicate "latched and locked" when that condition is not true.
In military aircraft, latch pins are used to hold each folding wing in place. These are locked by an independently-operated locking mechanism. The common method used to detect whether or not the latch pins are locked is a wing-mounted red "flag" that is driven above the wing contour by mechanical connections when the latch pins are unlocked. When they are locked, the flag is conversely driven below or flush with the wing contour. The pilot or flight crew determines the proper lock state by merely viewing the position of the flag. For high reliability, the flag is driven directly by the mechanism that locks the latch pins, with a minimum of interconnections. This necessitates physically locating the flag close to the locking mechanism.
Design safety for military aircraft is, by necessity, viewed differently than for civilian aircraft. Military operations typically require optimum airplane performance in order to provide the pilot with the best chance of survival. Military design anticipates that more pilots are lost as a result of combat than equipment failure. Hence, safety considerations are balanced with combat performance considerations. Since combat is not a consideration in the commercial arena, design safety takes on the utmost importance.
Although it has not been conclusively established, the above-described wing-locking technique used by the military may not provide the type of fail-safe operation that is required by the FAA for commercial aircraft, the FAA being the government agency that is responsible for closely regulating commercial aircraft design One known drawback to the military design is that the position of the flag does not necessarily provide an indication as to the integrity of the locking mechanism itself. That is to say, the flag may properly indicate that the wing's latch pins have been locked even though, in actual fact, the locking mechanism itself may be broken or otherwise damaged. Another drawback to the military design is that it is not particularly well-suited for most modern commercial jets, simply because the wings of such jets are not easily visible to the flight crew. Therefore, using a similar mechanical flagging technique in connection with the 777 wing would probably place an unacceptable operating burden on the airline that operates the plane.
The purpose of the present invention is to improve upon past latch pin and locking system designs used on military aircraft, and to produce a similar system that is highly reliable, and therefore suitable for civilian or commercial jet applications. How the invention accomplishes this, including the advantages it provides over and above prior art systems, will be explained below.